1. Field of Invention
This invention relates to apparatus and method for measuring arterial blood pressure. More particularly, the blood pressure monitor of the present invention allows noninvasive, instantaneous and continuous monitoring of blood pressure and provides for producing a waveform which closely represents the complete blood pressure waveform in a superficial artery.
Continuous, noninvasive blood pressure monitoring and recording are the subject of extensive investigation. Both tonometric and sphygmomanometric techniques are being considered. Where measurements are to be made while the subject is engaged in normal activities and over long periods of time tonometry is the preferred approach, because the pressure transducers used in tonometry can be made small, light and easy to wear, whereas the pressurized cuffs used in sphygmomanometry are relatively uncomfortable and cuff pressure changes required to obtain significant parameters are difficult to produce on a mobile subject. Further, the sphygmomanometer only yields the systolic (highest pressure in the arteries during contraction of the heart muscle) and diastolic (lowest pressure in the arteries during relaxation of the heart muscle) pressures, whereas the entire blood pressure waveform can be reproduced using the tonometric approach.
2. Relation to Prior Art
The invention builds on and represents an improvement on the technique described and claimed in U.S. Pat. No. 3,219,035, issued in the names of Gerald C. Pressman and Peter M. Newgard and assigned to the assignee of the present invention. The invention described in that patent relates to a force-balance technique which provides a noninvasive, precalibrated blood pressure measurement. The force-balance arrangement eliminates variations in blood pressure measurements which are only a result of variations in pressure between the transducer surface and the subject. A somewhat more complex approach to solving the same problem is found in U.S. Pat. Nos. 3,880,145 to Edward F. Blick and 3,123,068 to Robert P. Bigliano. A major difficulty encountered in the application of these devices has been locating an initial position of the measuring transducer element relative to the superficial artery and maintaining its position to obtain the accuracy required.
The invention described and claimed in copending patent application Ser. No. 848,753, filed even date herewith in the name of Peter M. Newgard and assigned to the assignee of the present invention, represents an improvement in the art in that it diminishes the positional accuracy required of the transducer array by at least an order of magnitude. Hence, initial placement of the device is trivial and the transducer design is such that it easily tolerates the nominal variations in position that are encountered during typical long term monitoring applications. Further, the method of selecting the proper waveform from among those produced by the array of individual transducers as set forth in that application is responsible, at least in part, for both diminution of required positional accuracy and tolerance for variations in position due to movements of the subject during monitoring of the pressure.
The present invention constitutes an improvement over both the art and the invention described and claimed in the Newgard application, supra, in that the method and means of selecting the particular pressure sensitive element of the array of transducers which most closely tracks the true blood pressure waveform and gives the most accurate measure of blood pressure are further refined.